1. Field of the Invention
The present invention relates to an active filter unit and, particularly, to an active filter unit used in a power transmission and distribution system for compensating harmonic currents.
2. Description of the Prior Art
FIG. 1 is a circuit diagram of the conventional active filter disclosed in the article entitled "Principle of Active Filter Compensation" in the proceeding of annual convention of the electric and information society, section 4, trend of active filter for power system, held in 1985.
In the figure, indicated by 1 is an a.c. power source, 2 is a harmonic current generating load coupled to the a.c. power source 1, 3 is a transformer, 4a-4d are transistor switches which constitute a single-phase bridge inverter connected across the secondary winding of the transformer 3, 5 is a capacitor, and 6 is an active filter constituted by the transformer 3, transistor switches 4a-4d, and capacitor 5.
Next, the operation will be described. The transistor switches 4a-4d are controlled in PWM (pulse width modulation) mode by a gate control circuit (not shown) so that they produce an a.c. output voltage E.sub.I with a synthesized waveform from a d.c. voltage E.sub.d charged in the capacitor 5, and the voltage E.sub.I is applied to the secondary winding of the transformer 3. FIG. 2 shows the equivalent circuit for this operation, in which indicated by 7 is an imaginary voltage source provided by the active filter 6, and 8 is the impedance of the transformer 3. For the fundamental-wave voltage E.sub.AC of the a.c. power source 1 and the a.c. output voltage E.sub.I of the active filter the following equation is established. EQU L(dI.sub.AF)/dt=E.sub.I -E.sub.AC (2)
where I.sub.AF is the output current of the active filter, and L is the impedance of the transformer 3.
The following harmonic current I.sub.H flowing through the load 2 is assumed. EQU I.sub.H =I.sub.N .multidot.sin(i N.omega.t) (2)
By controlling the active filter output current I.sub.AF to be equal to I.sub.H, it becomes possible that the current I.sub.S of the a.c. power source does not include a harmonic current.
In this case, equations (1) and (2) are reduced to the following. EQU N.omega.L.multidot.I.sub.N .multidot.cos(N.omega.t)=E.sub.I =E.sub.AC (3)
Accordingly, the active filter needs to provide the following a.c. output voltage E.sub.I. EQU E.sub.I =E.sub.AC +N.omega.L.multidot.I.sub.N .multidot.cos(i N.omega.t) (4)
The active filter needs to have the following output capacity VA. EQU VA=E.sub.I .multidot.I.sub.AF =E.sub.AC .multidot.I.sub.AF +N.omega.L.multidot.I.sub.N .multidot.cosw(i N.omega.t).multidot.I.sub.AF (5)
In equation (5), the first term on the right side represents the fundamental-wave capacity, and the second term represents the harmonic capacity. Usually the fundamental-wave capacity is greater than the harmonic capacity.
Conventional active filters are arranged as described above, and therefore necessitate control for both the fundamental-wave capacity and harmonic capacity. In case of compensating up to a higher-order harmonic current, the transistor switches 4a-4d need to be operated under high-frequency PWM control. The unnecessary high-frequency PWM operation involving the fundamental and low-order harmonic currents imposes an increased switching loss. Moreover, large capacity transistors operative in high-frequency switching are difficult to manufacture, and therefore manufacturing of a capacious active filter unit is also difficult.